From what I have heard the G50 is of superior design than the RUF (which makes sense since the G50 came much later).
G50 is also readily available and there are some shops that know how to adapt it to 930 without modifying the chassis.
In short, unless you really NEED the RUF 5 speed, there is no point in spending $$$ for it over a G50 conversion.

The RUf advantages are the lighter weight, the better gear ratios ( in mine ) for high performance use, the absolute vintage feel to the shift gate. shift lever location, the wonderful noises it makes and exclusivity as very few were ever made VS the more readily available parts/service of the G50.. A decision between the two I did not make as the RUF was already installed in the 1977 930 ( along with a long list of other upgraded items ) when I bought it many years ago from the Checkered Flag / Long Beach division.